Our invention comprises a control valve means for an hydraulicallY actuated clutch of an automatic motor vehicle transmission for reducing engagement force of a fluid pressure operated torque transmitting clutch when the transmission is conditioned for operation. It is particularly suited to be used in the automatic motor vehicle transmission disclosed in U.S. Pat. No. 4,744,269. Reference may be made to that patent to supplement this disclosure.
A pressure reducing control valve for an hydraulically actuated clutch of an automatic motor vehicle transmission is described in U.S. Pat. No. 4,265,346. That control valve is disposed in a control pressure line extending from a manual shift valve to the clutch. It comprises a valve member with two valve lands that register with internal valve lands in a valve chamber. Control pressure acts between the control lands. One end of the valve member is acted upon by a spring. The other end of the valve member is acted upon by pressure on the downstream side of a control orifice in a control pressure passage. A pressure buildup on the downstream side of the orifice during the initiation of the engagement of the clutch is delayed by means of a pressure accumulator. The accumulator pressure acts in the spring chamber to cushion the engagement of the clutch.
The control valve of the '346 patent requires for its operation an accumulator valve which normally occupies a relatively large space because of the volume of fluid involved. Available space, however, is extremely limited in control valve arrangements for automatic motor vehicle transmissions. In addition, the known control valve requires a regulating time for the clutch application pressure that is the same regardless of the degree of opening of the throttle valve for the engine. Even at the beginning of the pressure regulating period, the initial clutch application pressure is quite high. Following initial clutch engagement during the pressure regulating period a steep increase in pressure occurs.